1. Field of the Invention
The present invention relates to a switching power supply apparatus including multiple outputs and, more particularly, to a switching power supply apparatus capable of simultaneously controlling two outputs with high accuracy.
2. Description of the Related Art
Switching power supply apparatuses including multiple outputs are disclosed in, for example, Japanese Unexamined Utility Model Registration Application Publication No. 4-121181 and Japanese Unexamined Patent Application Publication No. 6-303771.
Japanese Unexamined Utility Model Registration Application Publication No. 4-121181 discloses a power supply circuit including two secondary windings in a transformer. In the power supply circuit disclosed in Japanese Unexamined Utility Model Registration Application Publication No. 4-121181, a rectifier smoothing circuit is provided for each of the secondary windings and the output voltage from one of the secondary windings is detected to perform feedback control.
Japanese Unexamined Patent Application Publication No. 6-303771 discloses a switching power supply circuit configured to have a current resonant converter. The switching power supply circuit disclosed in Japanese Unexamined Patent Application Publication No. 6-303771 is shown in FIG. 1. The switching power supply circuit is a current resonance type switching regulator including switching elements S1 and S2, a converter transformer 3, and a control circuit 2 controlling the switching elements S1 and S2. The converter transformer 3 includes a primary winding NP to which an input voltage is applied in synchronization with an on or off operation of the switching elements S1 and S2, a control voltage secondary winding NS2 for supplying an operating voltage VCC to the control circuit 2, and an output voltage secondary winding NS1 for extracting a direct current (DC) output V0. A full-wave rectifier circuit is provided at a secondary side of each of the output voltage secondary winding NS1 and the control voltage secondary winding NS2. The control circuit 2 complementarily drives the switching elements S1 and S2 at a duty ratio of 50% to control the output voltage Vo by frequency control.
Both of the power supply circuits disclosed Japanese Unexamined Utility Model Registration Application Publication No. 4-121181 and Japanese Unexamined Patent Application Publication No. 6-303771 are controlled so that the voltage of a rectifier smoothing output (at a control output side) of either of the two secondary windings in the transformer is stabilized. However, the output voltage is not determined only by a turn ratio of the windings in the transformer due to the effect of, for example, the degree of coupling of the transformer, resistance components, a forward drop voltage of a diode, and a surge caused by switching and the output voltage is varied with the magnitude of a load and/or a change or variation in characteristics of the elements at a non-control output side. In other words, there is a problem in that the accuracy of the voltage stabilization is low at the non-control output side.